Water Exchange on Manganese(III) Porphyrins. Mechanistic Insights Relevant for Oxygen Evolving Complex and Superoxide Dismutation Catalysis

In this work the rate constants (kex) and the activation parameters (ΔH‡, ΔS‡, and ΔV‡) for the water exchange process on MnIII centers have experimentally been determined using temperature and pressure dependent 17O NMR techniques. For the investigations the MnIII porphyrin complexes [MnIII(TPPS)S2]n− and [MnIII(TMpyP)S2]n+ (S = H2O and/or OH−) have been selected due to their high solution stability in a wide pH range, enabling the measurements of water exchange in the case of both diaqua and aqua-hydroxo complexes. We have experimentally demonstrated that the water exchange on MnIII porphyrins is a fast process (kex ≈ 107 s−1) of an Id to I mechanism, strongly influenced by a Jahn−Teller effect and as such almost independent of a porphyrin charge and a trans ligand. This is also supported by our DFT calculations which show only a slight difference in an average MnIII−OH2 bond found for a positively charged model porphyrin with protonated pyridine groups (2.446 A) and for a simple model without any subst...